Inertial navigational systems for aircraft usually employ a triad of accelerometers in combination with a triad of angular rate sensors for providing velocity and position information to a navigation computer. Specifically, from continuous knowledge of the vertical acceleration, i.e., perpendicular to the earth's surface, one can determine the vertical velocity and vertical position by time integration of the vertical acceleration as is well known.
Simple integration of vertical acceleration to provide vertical velocity, and integration of the vertical velocity to provide vertical position will result in the vertical axis information of an inertial navigator to have unbounded error due to two causes. First, error in the measurement of vertical acceleration (vertical acceleration being determined as a function of the three accelerometers) is directly integrated to cause a vertical velocity error and a vertical position error. Second, the erroneous vertical position error results in an error in the computer derived vertical-gravity component which, in turn, causes an even faster growth in vertical position error in the overall inertial navigation system.
To avoid the altitude diversion problem as just presented, it is common practice to blend the inertial altitude measurement from the primary control system, i.e. accelerometers and gyros, with the altitude derived from a barometric altimeter. The barometric altimeter being a well known device for providing altitude information as a direct function of the value of the barometric pressure.
In the prior art, the barometric altitude and the vertical position signal from the inertial system are compared to generate an altitude error signal. In turn, an acceleration correction or stabilization signal is derived as a function of the altitude error signal and used for modification of the inertial system vertical acceleration output before integration to determine vertical velocity. Further, a velocity correction or stabilization signal as a function of the altitude error signal is combined with the vertical velocity signal before integration to obtain the inertial vertical position. Although prior art control loops, constructed in a manner as just indicated, have been successful, they lack accuracy needed in some applications where the aircraft has to perform accurate flight patterns including rapid dive and climb operations.